Help to correct error

Hi All,
On opening this program for the first time, I get these errors listed
below. After the data entry and button press I get no error. Using IE
lastest version Tools menu item click to get Sun Java Console to see
this error.

I would appreciate help to correct "on opening up error"

TIA,
bH
----------------------------------------------------

Exception in thread "AWT-EventQueue-2"
java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at JustaTest.paint(JustaTest.java:33)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.seqPaintDirtyRegions(Unknown Source)
    at javax.swing.SystemEventQueueUtilities
$ComponentWorkRequest.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown
Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
program:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

//<applet code ="JustaTest.class" width=300 height=350></applet>

public class JustaTest extends JApplet implements ActionListener {
  Label introLbl = new Label(" Show A Circle");
  Label directionsLbl = new Label(" Enter #'s of Your Color and
Dimensions");
  JTextField NumbrColorTxtFld = new JTextField(4); // input color
number
  JTextField MeasDiaCircleTxtFld = new JTextField(4);
  Label choiceNumbrSidesLbl = new Label("Enter a 1(red),2(green),
3(blue)");
  Label measCircleLbl = new Label("Limit Entry to Measure > 0 and
<=150");
  Label calculateButtnLbl = new Label( "Click Button to Show ");
  JButton calculateButtn = new JButton("Paint It");

  public void init() {
    Container con = getContentPane();
    con.setLayout(new FlowLayout());
    add(introLbl);
    add(directionsLbl);
    add(choiceNumbrSidesLbl);
    add(NumbrColorTxtFld);
    add(measCircleLbl);
    add(MeasDiaCircleTxtFld);
    add(calculateButtnLbl);
    calculateButtn.addActionListener(this);
    add(calculateButtn);
  }

  public void paint(Graphics g) {
    super.paint(g);
    int colorNumber = Integer.parseInt(NumbrColorTxtFld.getText());
    System.out.println("NumbrColor = "+ colorNumber);
    int measureDia = Integer.parseInt(MeasDiaCircleTxtFld.getText());
    if ((measureDia<1)||(measureDia>150)){
      MeasDiaCircleTxtFld.setText("Oops!");
      measureDia = 3;
    }
    System.out.println("measureDia = "+ measureDia);
    switch (colorNumber){
      case 1:
        g.setColor(Color.red);
        break;
      case 2:
        g.setColor(Color.green);
        break;
      case 3:
        g.setColor(Color.blue);
    }
    g.drawOval(80,150,measureDia,measureDia);
    g.fillOval(80,150,measureDia,measureDia);
  }
  public void actionPerformed(ActionEvent thisEvent) {
    System.out.println("inside ActionEvent");
    Object source = thisEvent.getSource();
    if (source == calculateButtn){
    }
    repaint();
  }
}